An issue central to the study of behavior is the functional relationship between synaptic structure and the changes in synaptic effectiveness that accompany learning and memory. To address this problem we propose to examine the nature, extent and time course of the morphological events at identified synapses that accompany elementary forms of learning and memory and explore the role such structural alterations may play in initiating and maintaining the changes in synaptic function that underlie these behavioral modifications. Toward this end we plan to use a model system, the gill-withdrawal reflex of the marine mollusc Aplysia californica in which several forms of learning and memory have been studied to advantage on both the cellular and molecular level. We have recently exploited this system to examine the morphological basis of short- and long-term habituation and sensitization. Using horseradish peroxidase (HRP) to label the presynaptic terminals of identified sensory neurons (a critical site of plasticity for both forms of learning for both forms of learning) and complete serial reconstruction to analyze the total number and fine structure of snynaptic contacts, we have found that long-term memory is accompanied by structural alterations on two levels of synaptic organization: 1) changes in he number, size, and vesicle complement of focal regions of membrane specialization (active zones) of the synapse, and 2) a parallel but more dramatic and global trend involving modulation of the total number of synaptic varicosities. In contrast, the morphological correlates of short-term memory in Aplysia are restricted to shifts in vesicle populations associated with sensory neuron active zones. These findings provide the first direct evidence that behavioral modification produces structural changes at the level of identified synapses critically involved in learning and suggest a clear difference in the morphological events that accompany memories of differing durations. To explore these issues in more detail, we now plan to examine both the time course and underlying biochemical events responsible for the morphological changes at sensory neuron synapses that accompany short-and long-term memory. A temporal analysis should allow us to determine which class of structural change is necessary for the maintenance of memory and an analysis of mechanism will aid in establishing a more causal relationship between synaptic architecture and the learning process. Finally, to put these studies into perspective and to begin to examine possible mechanistic relationships with higher forms of learning, we also plan to conduct a morphometric analysis of associative learning. the approaches we have developed and the model system we plan to use provide the required specificity to address these problems directly and should increase our understanding of how the functional architecture of the synapse is related to its plastic capabilities.